In systems adapted to transmit or record sounds such as speech or music, it is often necessary to use electroacoustic apparatus that is directional in nature. With such apparatus only sounds emanating from preferred directions are converted into electrical signals while sounds from other directions are attenuated. In teleconferencing, array type microphones may be employed to pick up and transmit speech or other sounds from prescribed directions in large meeting rooms or auditoria so that background noise and extraneous sounds that may interfere with the intelligibility of the desired sounds are removed. Such array microphone structures may exhibit directable beam patterns that focus at talker locations and may be redirected to other points in the room as talker locations change. An arrangement that utilizes beam directional patterns is described in U.S. Pat. No. 4,485,484 issued Nov. 27, 1984.
One problem encountered with array type microphones relates to the modification of the shape of the directional pattern that occurs as the sound wave frequencies increase. As is well known in the art, the physical dimensions of a microphone array become larger compared to a wavelength in the medium as frequency increases. Consequently, the spatial directivity of an array is more acute at higher incident sound frequencies and the directional response pattern narrows with increasing frequency. This effect is especially true for widely utilized uniform arrays. The importnt frequency range of speech signals is generally greater than four octaves and the frequency range of musical sounds is wider. Thus, an array designed to have useful directivity at lower frequencies exhibits substantially more acute and practically less useful directivity at the high end of the sound frequency spectrum.
Prior art directive array microphone arrangements have been designed to provide a prescribed directional response pattern at a particular low range frequency and to provide an effective directional response pattern over a portion of the sound frequency spectrum. At higher frequencies, however, the aforementioned changes in directivity make the directional beam too narrow for practical purposes. As a result, the practically useful directional pattern of the array is only obtained over a limited portion of the audio frequency spectrum. It is an object of the invention to provide an improved electroacoustic transducer array having substantially constant directional response patterns over the audio spectrum.